There is a high demand for novel optically clear adhesives (OCA) for gap filling between an outer cover lens or sheet (based on glass, polyethylene terephthalate (PET), polycarbonate (PC), polymethyl methacrylate (PMMA), cyclic olefin copolymer, etc.) and an underlying display module of electronic display assembly. The presence of the OCA improves the performance of the display by reducing the refractive index mismatch between substrates and the air gap while also providing structural support to the assembly. Filling the gap with an index matching adhesive reduces sunlight and ambient light reflections inherent in the use of cover lenses or sheets. As a result, contrasts of conventional display panels are improved under ambient conditions.
During the manufacture of certain devices, such as liquid crystal displays (LCD), two rigid substrates, such as a LCD and a glass or polycarbonate lens, must be optically coupled by the OCA. Defects, such as patterns or creases introduced during the laminating process that also introduce stress and uneven pressures on the LCD, provide for unacceptable display appearance, often referred to as “mura.” On the lens side, the OCAs typically also have to be able to completely fill in the sharper corners of the decorative ink steps. To minimize this effect with OCAs, autoclaving (a process that includes exposure to high pressure, high temperature and time to enhance the OCA bond quality of the initial lamination step) is frequently employed.
First generation OCAs easily filled the LCD gap and after some work were able to conform to a low black ink-step (about 25 μm). However, mobile devices are now being manufactured using ink printed cover glass with a larger white ink-step (about 70 μm) to which current OCAs cannot conform well. Thus, there is a need for a new generation of OCA that is able to withstand the high temperature, high humidity (HTHH) testing conditions, while still flowing enough at autoclave temperatures (40-80° C.) to cover the larger ink-step dimensions. Additionally, there is a need for OCAs that are acid-free to eliminate conductive trace corrosion (like in a touch sensor) that can result from acid migration from the OCA.
While liquid OCAs are in essence infinitely compliant prior to curing (and essentially only have viscous character), they are not always easy to handle during assembly and caution has to be taken to minimize curing-induced stresses in the display; especially because after fully curing many of these materials are highly elastic. In contrast, sheet-like OCAs are free of curing-induced stresses, but they are also visco-elastic materials that can have difficulties overcoming larger ink steps or avoiding lamination stress during assembly.
Recently, the OCA visco-elastic property balance has been shifting more and more toward viscous character, but this can create issues with die cutting, die cut stability, and liner release. A certain amount of stiffness is needed from the OCA film to easily die-cut to the incredibly narrow (less than 0.15 mm) dimensional specifications of the customer and to be delivered without oozing or denting during shipping.
Furthermore, in the manufacture of OCA films in a continuous (on-web) coating and UV curing process starting from monomers or monomer syrups, it is currently necessary to use 1000 mJ/cm2 (or more) of UVA energy in order to prepare a highly cohesive adhesive film and to get high monomer conversion to polymer, or alternatively, less than 1000 mJ/cm2 can be used but at the expense of product quality and increased residual monomer content.